In addition to the contractile thick and thin filaments, pulmonary artery smooth muscle also contains significant numbers of another filament celled the intermediate (or 100 A) filament, the structure and function of which have not been fully characterized. Preliminary studies on intermediate filaments from other sources have suggested that they contain a unique protein of 55,000 molecular weight, and that the filaments play a cytoskeletal role in vivo. The principal objective of this research is to isolate and characterize the major protein component from pulmonary artery smooth muscle intermediate filaments in order that an understanding of filament structure, and ultimately, function can be obtained. Isolation of the principal protein of the filament will be achieved by classical separation techniques of protein chemistry, and the physico-chemical properties of this protein will be studied. In particular, it amino acid sequence will be determined and proposals for its secondary and tertiary structures will be developed. The interaction properties of this protein and its fragments will also be studied in order to identify the subunit assembly of the filament and to identify regions of the protein molecule involved in the assembly process. In addition to these studies, the protein composition of purified filaments will be examined to determine if other proteins are essential components of the native filament.